The present invention relates to the digital image processing arts. More particularly, the present invention relates to the preview of binary images compressed according to the CCITT standard without requiring complete decompression of the CCITT-compressed data, i.e., processing is performed in the CCITT-compressed domain. This allows for highly efficient generation of downsampled images such as thumbnail or other preview images.
Binary images of documents are often compressed using the CCITT standard. This is true, for example, in image printing and xerographic document reproduction apparatus. Presently, if one desires to preview/edit the image on a computer monitor or generate a thumbnail image of the compressed image, one is required to fully decompress the image data and perform processing to generate the preview image or thumbnail image in the decompressed domain. For example, a decompressed image defined at a resolution of 600 dots per inch (dpi) in both directions can be reduced or downsampled by a factor of 20 (i.e., to a resolution of 30 dpixc3x9730 dpi) by replacing each block of 20xc3x9720 pixels with the average of same. This is highly inefficient in terms of required processing time and resources.
In accordance with the present invention, a method for fast preview of CCITT-compressed digital images is provided. According to a first aspect of the invention, a method of deriving a preview image from CCITT-compressed digital image data that represent an original image is provided. The method comprises receiving CCITT-compressed image data that represent an original image having Nr rows and Nc columns so that the original image has a resolution of Nrxc3x97Nc. The image is reduced by factors of M1 and M2 along respective first and second axes to derive a preview image having Nr/M1 preview rows each having Nc/M2 pixel values so that the preview image has a resolution of Nr/M1xc3x97Nc/M2. The reducing step comprises, for each group of M1 rows of the original image: (i) reading the CCITT-compressed data that represent the group of M1 rows of the original image; (ii) for each constituent row in the group of M1 rows, deriving a location in the row for each transition represented in the CCITT-compressed data; (iii) for each row in the group of M1 rows, mapping each transition in the row to a pixel value in a reduced row having Nc/M2 pixel values whereby M1 reduced rows are defined; and, (iv) deriving a single preview row having Nc/M2 preview pixel values based upon the M1 reduced rows.
One advantage of the present invention resides in the provision of a method for fast preview of CCITT-compressed documents.
Another advantage of the present invention is found in the provision of a method for fast preview of CCITT-compressed documents that is faster than prior art methods and that utilizes less memory and other processing resources.
A further advantage of the present invention is the provision of a method for fast preview of CCITT-compressed documents wherein downsampling in the slow scan direction (inter-row) takes place after downsampling in the fast scan direction (intra-row) whereby the former operation is not computationally intensive.
Still other benefits and advantages of the invention will become apparent to those of ordinary skill in the art to which the invention pertains upon reading and understanding the following specification.